1. Field of the Invention
This invention relates to flowmeters and, more particularly, to flowmeters employed in detecting hydraulic system leaks, especially in aircraft.
2. Description of the Prior Art
The degree to which an aircraft (or any) hydraulic system leaks, externally or internally, is a direct indication of its general condition or state of repair. External leakage is very obvious, but the magnitude of internal leakage across lapped clearances, imperfect valve seats, defective seals, and the like is more difficult to determine. There are numerous mechanical, electrical, or electro-mechanical devices which measure internal leakage or flow, but they either fall short of desired performance, or are far more complex and expensive than necessary.
Hedland U.S. Pat. No. 3,805,611 discloses a flowmeter in which the position of an internal piston-like flow-sensitive member is tracked by means of a ferrous (magnetic) ring or armature which slides on an outer housing surface in following a permanent magnet on the inside of the device which is attached to the flow-sensitive member. The material (Alnico-type) from which the magnet is made is inherently very brittle and vulnerable to cracking. Because the magnet is in intimate contact with the system fluid, contamination of the system can readily occur. Such contamination is intolerable in aircraft systems, for example.
A similar structure used as an unloader indicator in a refrigeration system compressor of the screw type is disclosed in U.S. Pat. No. 3,738,116 of Gazda. Wermeille in U.S. Pat. No. 1,696,132 discloses a water meter using a pair of permanent magnets on opposite sides of a diaphragm seal with one of the magnets being driven by a pump and the other following the movement of the first magnet. As in all devices of the so-called "wet" magnet type, fluid flowing through such a device is subject to contamination from the flaking or break-up of the magnet in contact with the fluid.
Various types of differential pressure gages are disclosed in U.S. Pat. Nos. 2,948,151 of Astl, 3,125,062 of Raupp et al, 3,974,795 of Crisp, Jr., 3,980,040 and 4,014,284 of Read, 4,029,042 of Juhasz and 4,130,745 of Hetzer. However, these devices do not appear to be suitable for meaasuring fluid flow rates, as contrasted with indicating differences in two distinct applied pressures.